1. Field of the Invention
The present invention relates to an optical modulator and an optical modulation method, and more particularly, the present invention relates to an optical modulator and an optical modulation method for obtaining an optical intensity as a result of modulation by an alternating-current signal of several hundreds or lower megahertz.
2. Description of the Related Art
Conventionally, optical modulators have been configured such that light passes through a magnetic ferrite single crystal, such as an yttrium iron garnet (YIG), a thin-line transducer, a microstrip line, or other suitable element, microwaves are applied thereto, and the optical intensity is modulated by the microwaves. (For example, refer to an article by Makoto Tsutsumi, Tetsuya Ueda, and Others in xe2x80x9cShingaku Gihoxe2x80x9d, Vol. 98, No. 123 MW98-41, OPE98-33 (1988), pp. 45). A conventional optical modulator is shown in FIG. 1.
FIG. 1 is a view showing an example of a conventional optical modulator that relates to the present invention described hereinbelow. An optical modulator 10 shown in FIG. 1 includes a plate-like magnetic garnet single crystal 12 for use as a magnetic ferrite single crystal. A microstrip line 14 defining a transducer is provided in a central portion of one of the primary surfaces of the magnetic garnet single crystal 12.
An output terminal of a microwave-signal generator 16 is connected to one end of the microstrip line 14 via(a microwave amplifier 18. In addition, an output terminal of an optional-signal generator 19 is connected to the microwave-signal generator 16. The optional-signal generator 19 modulates a microwave that has been output from the output terminal of the microwave-signal generator 16, and it uses an alternating-current signal having a frequency that is lower than that of the microwave for the modulation. The other end of the microstrip line 14 is connected to a terminal resistor 20.
In addition, a permanent magnet (not shown) is provided near the magnetic garnet single crystal 12. The permanent magnet is used to apply a dielectric-current magnetic field in a direction that is parallel to the magnetic garnet single crystal 12 and perpendicular to the microstrip line 14.
Outside of one peripheral surface of the magnetic garnet single crystal 12, a light source such as a laser-beam source 22, a polarizer 24, and a first lens 26 are arranged to be close to the magnetic garnet single crystal 12 in that order. The laser-beam source 22 generates laser beams. The polarizer 24 linearly polarizes laser beams, which have been generated by the laser-beam source 22, in a predetermined direction. The first lens 26 concentrates the laser beams, which have been generated by the laser-beam source 22, into the magnetic garnet single crystal 12. In this way, the laser-beam source 22 is arranged such that laser beam emitted from the laserbeam source 22 is introduced to the magnetic garnet single crystal 12 and modulated by the microwave applied to the microstrip line 14.
In addition, in the outside of another peripheral surface of the magnetic garnet single crystal 12, a second lens 28, an analyzer 30, a third lens 32, and a photodiode 34 are arranged to be spaced apart from the magnetic garnet single crystal 12 in that order and arranged to receive the light beam emitted from the magnetic garnet single crystal 12. The second lens 28 rectifies laser beams transmitted through the magnetic garnet single crystal 12 to parallel beams. The analyzer 30 allows the linearly polarized laser beams to be transmitted in a predetermined direction and the analyzer is arranged to have the crossed-Nicols relationship with the polarizer 24. The third lens 32 converges the laser beams transmitted through the analyzer 30. The photodiode 34 detects laser-beam signals.
Also, an output terminal of the photodiode 34 is connected to an input terminal of a photoelectric-current amplifier 36.
In the optical modulator 10 shown in FIG. 1, optical systems (configurations of the magnetic garnet single crystal 12 and the components on two sides thereof) are substantially the same as a measurement optical system for transmission-type photomagnetic effects, such as a Faraday effect. Specifically, a microwave is applied to the magnetic garnet single crystal 12 via the microstrip line 14 defining the transducer to couple the light and the microwave together in the magnetic garnet single crystal 12, and a polarized state of the light is modulated according to the microwave. The modulated state is then converted by a Faraday optical system into the variation in the intensity for the implementing detection.
FIG. 2 is a view showing a waveform of an optical-signal that is produced without a microwave being applied in the optical modulator 10 shown in FIG. 1. FIG. 3 is a view showing a waveform of an optical-signal that is produced with a microwave being applied in the optical modulator 10 shown in FIG. 1. When the microwave is not applied, as shown in FIG. 2, the light transmitted through the magnetic garnet single crystal 12 remains in the state of direct current, and the optical output is constant. However, when the microwave is applied, as shown in FIG. 3, a modulated alternating-current component overlaps with a direct-current component.
The microwave applied to the magnetic garnet single crystal 12 is preliminarily modulated according to the alternating-current signal, which has been generated by the optional-signal generator 19 and which has a frequency lower than that of the microwave. Therefore, the intensity of the light led to be incident on the photodiode 34 is also modulated further by the microwave modulated according to the low-frequency signal. When the frequency band of each of the photoreceptor systems (such as the photodiode 34 and the photoelectric-current amplifier 36) includes the frequency of the signal that modulates the microwave and is lower than the frequency of the microwave, the individual photoreceptor system functions as a low-pass filter in which only the signal for modulating the microwave has sensitivity to the optical signal. Accordingly, the alternating-current component of the optical signal that is output from the photoelectric-current amplifier 36 matches the low-frequency signal that modulates the microwave. As a result, the optical-signal output is modulated according to the alternating-current signal having the frequency lower than the microwave.
The reason that the microwave is first modulated using the low-frequency alternating-current signal and is then applied to the magnetic garnet single crystal 12 is that any frequency band other than the microwave frequency band cannot be propagated through the magnetic garnet single crystal 12.
In the optical modulator 10, either when an external magnetic field is not applied to the magnetic garnet single crystal 12 or when an applied external magnetic field applied thereto is extremely weak, the microwave is just propagated therethrough, and optical modulation is implemented by the microwave. When a sufficiently intensive external magnetic field is applied to the magnetic garnet single crystal 12, a static magnetowave is excited, and optical modulation is implemented by the static magnetowave.
However, conventionally, while attempts have been made to generate an optical modulation phenomenon according to either the microwave or the static magnetowave, no attempt has been successfully made to achieve greatly increased modulation.
For this reason, with the conventional optical modulator having the configuration shown in FIG. 1, problems are caused such that the optical-modulation amplitude cannot be easily increased to a desired very large value, and the signal to noise ratio is very small.
In order to overcome the problems described above, preferred embodiments of the present invention provide an optical modulator optical and a modulation method in which the optical modulation amplitude is greatly increased and in addition the signal to noise ratio (S/N) is greatly increased.
According to a preferred embodiment of the present invention, an optical modulator includes a magnetic ferrite single crystal, an optical source, a photoreceptor system and an analyzer. The magnetic ferrite single crystal has a transducer mounted thereon and arranged to receive a microwave. The microwave is modulated by a signal having a frequency lower than the microwave. The optical source is arranged such that light emitted from the optical source is introduced to the magnetic garnet single crystal and modulated.by the microwave applied to the transducer. The photoreceptor system is arranged so as to receive the modulated light that is emitted from the magnetic ferrite single crystal. The analyzer is provided between the magnetic ferrite single crystal and the photoreceptor system, and the analyzer is arranged such that a rotation angle of the analyzer about an optical axis thereof is shifted by an angle in the range of about 40 degrees to about 50 degrees from an extinct position at which an amount of direct-current light transmitted through the analyzer is minimized.
In the optical modulator of one of the preferred embodiments of the present invention, a frequency band of a photoreceptor system includes, for example, the frequency of a signal that modulates the microwave, and concurrently, is lower than the frequency of the microwave.
In the optical modulator of another preferred embodiment the present invention, the magnetic ferrite single crystal is, for example, an iron yttrium garnet.
An optical modulation method according to another preferred embodiment of the present invention applies a microwave to a transducer formed in a magnetic ferrite single crystal and thereby modulates light being transmitted through the magnetic ferrite single crystal. The method is performed such that the microwave applied is modulated by an alternating-current signal having a frequency that is lower than that of the microwave, and the rotation angle of an analyzer in the optical-axis rotation direction is set by further rotating the analyzer by about 40 degrees to about 50 degrees from a rotation angle (extinct position) at which the amount of direct-current light transmitted through the analyzer is minimized.
In the optical modulation method of a preferred embodiment of the present invention, a frequency band of a photoreceptor system includes, for example, the frequency of a signal that modulates the microwave, and concurrently, is lower than the frequency of the microwave.
In the optical modulation method of another preferred embodiment of the present invention, the magnetic ferrite single crystal is, for example, an iron yttrium garnet.
In the optical modulator and the optical modulation method according to various preferred embodiments of the present invention, the rotation angle in the optical-axis rotation direction of the analyzer is set preferably by further rotating the analyzer by about 40 to about 50 degrees from a rotation angle (extinct position) at which the amount of direct-current light transmitted through the analyzer is minimized. Thereby, substantially the largest signal to noise ratio (S/N) can be obtained.
According to a conventional technique, the opticalmodulation amplitude of an output from the first photoelectric-current amplifier 36 is 1 xcexcV. However, preferred embodiments of the present invention achieve a significant advantage of increasing the optical-modulation amplitude to about 10 mV.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments thereof with reference to the attached drawings.
For the purpose of illustrating the invention, there is shown in the drawings several forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.